Certain geological formations contain petroleum that companies would like to extract. However, finding the formations that contain commercially viable petroleum reservoirs can be challenging. Even if a company knows the general location of a petroleum reservoir, determining preferred drilling sites, drilling depths, and extraction strategies can be difficult with conventional technologies. Compounding the situation, oil-bearing geological formations are often beneath the ocean or some other body of water.
The conventional technologies that are available for detecting petroleum and identifying geological structures relevant to petroleum extraction are particularly limited when the petroleum and geology are located underwater. Improved technologies are needed to evaluate underwater geology in support of placing petroleum wells and conducting other operations aimed at locating and obtaining petroleum.
The marine controlled source electromagnetic (“CSEM”) geophysical survey method typically uses a towed bipole source and deployed ocean-bottom receivers for mapping sub-seafloor resistivity variations (for example, U.S. Pat. No. 6,522,146 to Srnka, PCT Patent Application Publication No. WO 2003/048812 (Macgregor and Sinha), U.S. Pat. No. 6,628,119 to Eidesmo et al., U.S. Pat. No. 5,770,945 to Constable, and PCT Patent Application Publication No. WO 2004/053528 (Constable)).
In some applications, however, a towed inline receiver can be deployed for improved efficiency; see, for example, U.S. Pat. No. 7,038,456 to Ellingsrud et al. The subsequent inversion of the electromagnetic data for earth resistivity (resistivity anomalies are hydrocarbon indicators) would be further improved if multi-component receivers could be used. The improvements would be even more substantial if a broadside source were also available. For this latter case, the additional receiver components (particularly the crossline component) essentially permit simultaneous collection of inline and broadside data in a towed-receiver configuration. This additional information improves the discrimination of resistive reservoirs from changes in background resistivity.
In U.S. Pat. No. 7,038,456, use of a towed receiver is disclosed including the possibility of a two-component receiver with the receivers inline and crossline to the tow direction. Positioning the two receiver components oblique to the tow direction is also mentioned. A two-component towed source with 90° phase difference between the components is discussed in PCT Patent Application Publication WO 2006/059122. This configuration would produce a circularly polarized source. The combination of the circularly polarized source with the at least two-component receiver would give the opportunity for the simultaneous collection of inline and broadside data.
PCT Patent Application Publication No. WO 2008/008127 discloses using adjustable vanes or wings to control position and orientation of a towed streamer of electromagnetic or seismic receivers.